1. Field of the Invention
The present invention relates to a warm-up control device for a machine tool.
2. Description of the Related Art
The main shaft attached to a machine tool is rotated by a rotary drive means such as a motor. Therefore, it is known that a machining tool attached to a tip of the main shaft is displaced by the heat generated from the rotary drive means itself or heat generated from a main shaft bearing, causing errors. Moreover, when thermal displacement occurs in a base of the machine tool due to the heat generated by drive means such as a motor used for axis feeding for relatively displacing the positions of a workpiece and the tool, changes in temperature in the environment where the machine tool is installed, and the heat generated in driving of the abovementioned main shaft, the relative position between the workpiece and the tool becomes shifted, causing machining errors.
A method called “warm-up” has been known as a way to solve these problems. A warm-up is an operating method in which, especially due to the impact of thermal displacement of the main shaft caused by its rotation, the main shaft is idled in order to maintain machining accuracy, and then a workpiece is machined after thermal displacement of the main shaft becomes stable. Although the idling for achieving stable machining accuracy is performed until the thermal displacement in the machine tool is stabilized, how long the idling needs to be performed is determined based on the experiences or intuition of a skilled operator. Thus, whether the thermal displacement is definitely stabilized or not is uncertain. Moreover, it might take an unnecessarily long time for such warm-up.
Japanese Patent Application Laid-Open No. 2009-113138, for example, discloses a method that continuously measures the position of the tip of a tool by using a measuring device such as a laser interferometer, calculates the thermal displacement amount of the position of the tip of the tool from a result of the measurement, and ends the warm-up based on the calculated thermal displacement amount.
However, Japanese Patent Application Laid-Open No. 2009-113138 only describes that the way of ending the warm-up is to simply end the warm-up or to automatically start machining. When the warm-up is simply ended, the machining starting point on the tool at which the machining is started needs to be reset manually. In a case where an operator gets caught up in another work and leaves the machine tool after the end of the warm-up, the warm-up needs to be carried out again, which is not necessarily efficient. It is not particularly problematic to start machining automatically, but automatically resetting the machining starting point requires a measuring device that is capable of controlling the tool by using a numerical controller. However, not all machine tools are equipped with such measuring devices.